Next, using drawings, prior art is hereunder described. FIG. 7 illustrates one example of an afocal system on a conventional optical head. FIG. 8 schematically illustrates one example of a photodetector in FIG. 7. FIG. 9 schematically illustrates another example of a photodetector in FIG. 7. FIG. 10 schematically illustrates one example of a focal system on a conventional optical head.
First, using FIG. 7, and example of an afocal system on a conventional optical head is hereunder described. In this figure, numeral 1 represents a laser diode that serves as a light source; 2, collimator lens that converts the beam from the laser diode 1 into a parallel beam; 3, beam splitter that has an optical splitting surface 3a for separating the incident beam into two independent beams; 4, objective lens that shifts in the arrow I direction during a focusing operation, and travels in the vertical direction relative to the plane of this figure, during a tracking operation, and that focuses the parallel beam from the beam splitter 3 onto a recording medium 5; 6, photodetector disposed on the side face of the beam splitter 3. In moving to the reading position, the optical head as one entity moves in the vertical direction relative to the plane of this figure.
According to such a constitution, the beam emitted from the laser diode 1 is directed to the recording medium 5 surface via the collimator lens 2, beam splitter 3, and objective lens 4. The beam reflected from the recording medium 5 is separated by the optical splitting surface 3a of the beam splitter 3, and one independent beam is emitted substantially vertically relative to the optical splitting surface 3b and is directed to the photodetector 6. As shown in this figure, when the recording medium 5 is in position (1) (indicated by a solid line), the optimum focal relation is present between the recording medium 5 and the objective lens 4, wherein the fed-back beam is indicated by thin lines. When the recording medium 5 is in position (2) (indicated by a two dot chain line), the recording medium is too near to the objective lens 4, wherein the fed-back beam is indicated by dashed lines. When the recording medium 5 is in position (3) (indicated by a two dot chain line), the recording medium is too far from the objective lens 4, wherein the fed-back beam is indicated by alternate long and short dashed lines.
Next, based on FIG. 8, the photodetector 6 is hereunder described. According to this figure, the photodector 6 has two concentric optical splitting surfaces 6a and 6b. Numeral 7 represents a differential amplifier that, when fed with outputs A and B from the respective optical splitting surfaces 6a and 6b, performs arithmetical operation "A-B". As can be understood from this figure, the focal status of the objective lens 4 varies the diameter of the beam directed to the photodetector 6. Accordingly, the output level "A-B" on the differential amplifier 7 correspondingly varies, thereby the output level is used as a focal point detecting signal (focusing error signal).
Next, another example of a photodetector is described using FIG. 9. In this figure, numeral 8 represents a photodetector that has three optical splitting surfaces 8a, 8b, and 8c. Numeral 9 represents a differential amplifier that is fed with the outputs A, B, and C, correspondingly of respective optical splitting surfaces 8a, 8b, and 8c, and that performs an arithmetic operation "(A+C)-B". According to this example too, in a manner identical with the preceding example, the diameter of the beam directed to the photodetector 8 varies depending on the focal status of an objective lens 4. Correspondingly, the output level "(A+C)-B" of the differential amplifier 9 varies, and is used as a focusing error signal. Incidentally, the focal point detecting technique (focus error detecting technique) shown in FIGS. 8 and 9 is known as a beam size technique.
Based on FIG. 10, one example of an afocal system on a conventional optical head is hereunder described.
In this figure, numeral 11 represents a laser diode that serves as a light source; 12, beam splitter than has an optical splitting surface 12a for separating the incident beam into two independent beams; 13, objective lens that shifts to the arrow I direction during a focusing operation, and travels to the vertical direction relative to the this figure, during a tracking operation, and that focuses the non-parallel beam from the beam splitter 12 onto a recording medium 14; 15, photodetector disposed on the side face of the beam splitter 12. In moving to the reading position, the optical head as one entity moves to the vertical direction relative to this figure.
According to such a constitution, the beam emitted from the laser diode 11 is directed to the recording medium 14 surface via the beam splitter 12 and objective lens 13. The beam reflected from the recording medium 14 is separated by the optical splitting surface 12a of the beam splitter 12, and one independent beam is emitted from an optical splitting surface 12b and is directed to the photodetector 15. As shown in this figure, when the recording medium 14 is in position (1) (indicated by a solid line), the optimum focal relation is present between the recording medium 14 and the objective lens 13, wherein the fed-back beam is indicated by thin lines. When the recording medium 14 is in position (2) (indicated by a two dot chain line), the recording medium is too near to the objective lens 13, wherein the fed-back beam is indicated by dashed lines. When the recording medium 14 is in position (3) (indicated by a two dot chain line), the recording medium is too far from the objective lens 4, wherein the fed-beck beam is indicated by alternate long and short dashed lines.
The focal point detecting technique used in conjunction with the photodetector 15 is a beam size technique the same as that of the optical head on the afocal optical system mentioned previously, and the description of which is omitted.
According to the so-constituted prior art examples, the beam size technique is advantageous in that as compared with other focal point detecting techniques (such as an astigmatism technique, knife-edge technique, critical angle technique, and Foucault technique), this technique entails simple constitution, and fewer parts number, and lower manufacturing cost. Being so simply constituted, and having fewer parts, an optical head based on this technique is light and compact, and that allows high speed accessing. This technique, however, incurs low focal sensitivity.
At the same time, since not requiring a collimator lens, an optical head on the focal system such as in FIG. 10 is simply constituted, having fewer parts, and lower manufacturing cost, as compared with the optical head of the afocal system such as that shown in FIG. 7. Additionally, being so simply constituted, and having fewer parts, an optical head based on this focal system is light and compact, and that allows high speed accessing. This technique, however, incurs a problem that since light emitted from a laser diode 11 travels in the form of divergent light through a beam splitter 12 an aberration is produced, and satisfactorily converging the laser beam on the recording medium 14 is difficult. Another problem is that with a focal optical system, adjusting respective optical elements is difficult.